1. Field of the Invention
The present invention relates to a spectrophotometer for use as a detector for detecting sample components separated by, for example, a liquid chromatograph.
2. Description of the Related Art
A fluorospectrophotometer generally includes a sample cell, a photodetection unit, a light source, an excitation optical system, and a fluorescence optical system. An example of the sample cell includes a flow cell to be arranged downstream from an analytical column of a liquid chromatograph to allow an eluate containing sample components separated by the analytical column to flow through it. The excitation optical system is provided to extract an excitation light component from light emitted from the light source and deliver the excitation light component to the sample cell. A sample flowing through the sample cell is excited by the excitation light component and emits fluorescence. The fluorescence optical system is provided to extract a fluorescence component from light emitted from the sample cell and deliver the fluorescence component to the photodetection unit.
The photodetection unit generally has a photomultiplier tube. The photomultiplier tube produces electrons according to the amount of light incident on its entrance window due to photoelectric effect, multiplies the number of electrons, and outputs electrical current. When light enters the photomultiplier tube to which a constant voltage is being applied to keep its multiplication factor constant, electrical current of which the magnitude is proportional to the amount of light incident on the entrance window is outputted. Therefore, the amount of light incident on the entrance window can be measured by detecting the value of the output current. When the multiplication factor of the photomultiplier tube is higher, the number of electrons proportional to the amount of light incident on the photomultiplier tube is more greatly multiplied and a larger current value is outputted so that higher detection sensitivity is achieved.
However, such a photomultiplier tube has a problem that it is deteriorated when electrical current exceeding a maximum allowable limit passes through it. For example, in a case where the wavelength of excitation light and the wavelength of fluorescence are the same or close to each other, the excitation light reflected by a sample is not attenuated at all in the fluorescence optical system, or even when it is attenuated in the fluorescence optical system, the degree of attenuation is not so great. In this case, the excitation light component as well as the fluorescence emitted from the sample enters the photomultiplier tube and, therefore, the amount of light incident on the photomultiplier tube is increased, which may result in the passage of excessive electrical current through the photomultiplier tube leading to the deterioration of the photomultiplier tube.
In order to solve the above problem, a method for decreasing electrical current passing through a photomultiplier tube has been proposed (see, for example, Japanese Patent Application Laid-open No. 2000-9644). More specifically, the limit-current-value of a photomultiplier tube is defined as a threshold value, and electrical current outputted from the photomultiplier tube is compared with the threshold value. When the output electrical current exceeds the threshold value, a voltage applied to the photomultiplier tube is decreased to reduce the multiplication factor of the photomultiplier tube so that electrical current passing through the photomultiplier tube is decreased. It is to be noted that the limit-current-value of the photomultiplier tube refers to a maximum level of electrical current that can pass through the photomultiplier tube without deteriorating the photomultiplier tube.
However, in the case of using such a method, since a voltage applied to the photomultiplier tube is decreased after electrical current outputted from the photomultiplier tube exceeds the limit-current-value, electrical current exceeding the limit-current-value passes through the photomultiplier tube for any length of time, and therefore, the deterioration of the photomultiplier tube is to some degree inevitable. Even in the case of using this method, the multiplication factor of the photomultiplier tube can be reduced before electrical current passing through the photomultiplier tube exceeds the limit-current-value by selecting a threshold value lower than the limit-current-value. However, in this case, it is impossible to maximize the sensitivity of the photomultiplier tube.